[libc++] TAKE 2: Make system libraries PRIVATE dependencies of libc++

We tried doing that previously (in r373487) and failed (reverted in
r373506) because the benchmarks needed to link against system libraries
and relied on libc++'s dependencies being propagated. Now that this has
been fixed (in r374053), this commit marks the system libraries as
PRIVATE dependencies of libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374056 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed